The invention described herein relates to the disposal of radioactive waste and more particularly to a method of making hydraulic cement thixopastes which encapsulate the radioactive material.
The heat exchange coolant circulated through the primary and secondary loops of a nuclear reactor during the course of its operation, accumulates and carries radioactive matter which must be removed from the coolant system. This matter which constitutes harmful waste products consists of both dissolved and solid particulate material, such as that resulting from ion exchange scrubbing of waste waters, leaking fuel rods, corrosion of metallic parts, sludges from filters and evaporators, and the like, together with other dissolved impurities, all of which have been irradiated and take on radioactive characteristics.
In the past, removal of these radioactive byproducts (radwaste) from the coolant system has been effected by mechanical filters of different types, chemical precipitating agents, evaporators and demineralizers, and other means capable of capturing and retaining particles often microscopic in size. The capturing agents or mediums together with the radioactive materials which often contain boron were mixed with other more concentrated liquids or solids, such as glass, or disposed of by encapsulating the resultant product in cement or cement lined steel tanks suitably designed for long-term disposal. It has been considered that an ideal method of encapsulating these materials together with other radioactive wastes from laboratories, field operations, and the like, was to mix them with cement and store the cement-waste product in oil drums of 55 gallon capacity. However, attempts to encapsulate the borated waste coolant from light water reactors in hydraulic cements have presented problems associated with variable retardation of set in in the cement mixture, segregation of material within the encapsulation container and the formation or release of free liquid in the form of bleed water.
The borates present in the waste coolant retard the setting of cements by forming calcium borates on the hydrating surface of the cement particles, thereby blocking calcium silicate hydration and the double salt formation reaction between sulphate and aluminates. As a result of the unwanted precipitation reaction, there is a delay in the setting time and a retardation in the rate of strength development, together with the segregation of liquid from the paste as sedimentation takes place under the influence of gravity. Evidence of the delay in setting time can be seen in the heat evolution rate curves of FIG. 1 wherein the hydration pattern of cement paste made with water is compared with the pattern from the same paste made with saturated borax solution.